1. Field of the Invention
This invention relates to zoom objectives, and, more particularly, to compact zoom objectives suited (to photographic optical systems) for still cameras, 8 m/m cine cameras, video cameras, etc.
2. Description of the Prior Art
In the past, there have been many proposals for zoom objectives composed of four lens components (or groups), or so-called 4-component zoom objectives. This 4-component zoom objective is generally composed of the 1st lens component counting from the object side and is stationary during zooming and axially movable for focusing; the 2nd component is axially movable for varying the overall focal length; the 3rd component is axially movable for compensating the image shift resulting from the movement of the 2nd component; and the 4th component is stationary during zooming and forms an image of magnification satisfying the overall focal length. In such a type of zoom objective, the 2nd lens component mainly performs the magnification varying effect. Since the total zooming movement is determined by the focal length of the 2nd lens component, therefore, the increase in the zooming ratio calls for an increase of the total movement of the 2nd lens component. For this reason, a large space for the movement of 2nd and 3rd lens components is required, thus producing a drawback that the total length of the lens system is unavoidably increased. Another drawback of the zoom objective of the type described is that, as the zoom ratio increases, the physical dimensions of any part of the overall lens system must be increased.
Attempts have been made to improve these drawbacks so that it is possible to design a zoom objective of an increased zoom ratio while still permitting a minimization of the bulk and size of the entire lens system by means such as disclosed in Japanese Patent Publication No. Sho 41-13667 (U.S. Pat. No. 3,481,666). According to this prior art, the lens system comprises, from front to rear, a 1st lens group of positive power, a 2nd lens group of negative power, a 3rd lens group of positive power and a 4th lens group of positive or negative power, whereby when zooming from the wide angle to the telephoto position, the 1st and 3rd lens groups move forward as a unit, while the 2nd lens group moves rearward. However, this type of zoom objective becomes impossible to maintain the image plane constant where the image magnification of the 2nd lens group exceeds unity, or the absolute value of image magnification becomes not less than 1, and, therefore, to provide an optical system of constant image plane. This fact has first been found out by the applicant of this patent application. This is the very essential cause which prohibits increase of the zoom ratio of this type of zoom objective and further reduction of the bulk and size of the lens system.
Other zoom objectives comprising four lens groups of which three are moved to effect zooming, include those specified in U.S. Pat. Nos. 4,240,700 and 4,306,776.